Generally, a secondary battery refers to a rechargeable battery, while a primary battery refers to a non-rechargeable battery. Secondary batteries are widely used for electric vehicles or electronic devices such as cellular phones, notebook computers, video cameras or the like. In particular, a lithium secondary battery has an operating voltage of about 3.6 V, triple the capacity of nickel-cadmium batteries or nickel hydrogen batteries generally used as power sources of electronic devices, and due to its high energy density per unit weight, are being utilized more and more.
The lithium secondary battery generally uses lithium oxide and carbonaceous material as cathode active material and anode active material, respectively. The lithium secondary battery includes a cell assembly configured by integrating unit cells, in each of which a cathode plate and an anode plate respectively coated with the cathode active material and the anode active material are disposed with a separator being interposed between them, and an exterior case which seals and accommodates the cell assembly together with an electrolytic solution.
Depending on the shape of the battery case, the lithium secondary battery may be classified into a can type secondary battery in which the cell assembly is included in a metal can and a pouch type battery in which the cell assembly is included in a pouch case of an aluminum laminate sheet.
The pouch-type secondary battery has low production costs and high energy density, and a large-capacity battery pack may be easily configured by connecting such pouch-type secondary batteries in series or in parallel. For this reason, the pouch-type secondary battery is in the spotlight as a power source of an electric vehicle or a hybrid vehicle.
In the pouch-type secondary battery, a cell assembly connected to a plate-shaped electrode lead is sealed in the pouch case together with an electrolytic solution. The electrode lead is partially exposed out of the pouch case, and the exposed portion of the electrode lead is electrically connected to a device to which a secondary battery is mounted or used for electrically connecting secondary batteries to each other.
FIG. 1 is an exploded perspective view showing a conventional pouch-type lithium secondary battery, and FIG. 2 is a perspective view showing an appearance of the conventional pouch-type lithium secondary battery.
Referring to FIGS. 1 and 2, the conventional pouch-type lithium secondary battery 10 includes an electrode assembly 30, a plurality of electrode tabs 40, 50 extending from the electrode assembly 30, electrode leads 60, 70 welded to the electrode tabs 40, 50, and a pouch package 20 accommodating the electrode assembly 30. The electrode assembly 30 is a power generation unit in which a cathode and an anode are stacked in order with a separator being interposed between them. The electrode assembly may have a stacking structure, a jelly-roll structure or a stacking/folding structure.
A secondary battery including the jelly-roll type electrode assembly 30 is disclosed in, for example, Korean Unexamined Patent Publication No. 2009-88761 (entitled “Secondary battery including a jelly-roll type electrode assembly”) and Korean Unexamined Patent Publication No. 2007-47377 (entitled “Rectangular secondary battery including a jelly-roll type electrode assembly”). In addition, the electrode assembly 30 of a stacking/folding structure or a secondary battery including the electrode assembly 30 are disclosed in, for example, Korean Unexamined Patent Publication No. 2008-36250 (entitled “Mixed stacking and folding electrode assembly and a secondary battery having the same) and Korean Patent Registration No. 0987300 (entitled “Stacking-folding electrode assembly and its manufacturing method).
The electrode tabs 40, 50 respectively extend from corresponding electrode plates of the electrode assembly 30. The electrode leads 60, 70 are electrically connected by welding to a plurality of electrode tabs 40, 50 extending from each electrode plate and coupled to the pouch package 20 so that they are partially exposed outwards. The pouch package 20 is made of a soft package material such as an aluminum laminate sheet. The pouch package 20 has a space in which the electrode assembly 30 may be accommodated, and has an overall pouch shape.
When welding the electrode tabs 40, 50 and the electrode leads 60, 70, an ultrasonic welding technique ensuring a good heat-affected zone (HAZ) and easily applied to weld a thin metal foil is generally used. The ultrasonic welding generates ultrasonic vibrations of 10 kHz to 75 kHz and welds metals by using frictional heat caused by the ultrasonic vibrations between the metals. In other words, if an ultrasonic welding machine applies ultrasonic vibrations to the electrode tabs 40, 50 and the electrode leads 60, 70 which are in contact, frictional heat is generated at the contact surface between the electrode tabs 40, 50 and the electrode leads 60, 70, and the electrode tabs 40, 50 and the electrode leads 60, 70 are welded by means of the frictional heat.
Meanwhile, the cathode structures 40, 60 and the anode structures 50, 70 are generally made of materials with different properties. Here, the cathode structures 40, 60 are generally made of aluminum and the anode structures 50, 70 are generally made of copper or nickel-plated copper. In other words, the cathode tab 40 and the cathode lead 60 are made of aluminum, and the anode tab 50 and the anode lead 70 are made of copper or nickel-plated copper.
In the conventional lithium secondary battery 10, aluminum is used for the cathode as described above. At this time, aluminum has an oxidation potential of 1.39V and is unstable in a thermodynamic aspect. For this reason, aluminum oxide Al2O3 stable in a thermodynamic aspect is generally applied to the surface of the cathode. This oxide is stable in the air and thus has a resistance against a corrosive reaction. However, aluminum oxide does not have electric conductivity since it has no free electrons. Therefore, even though the cathode lead and the cathode tab are surface-treated to solve the above problem in the conventional art, resistance increases due to low electric conductivity, which causes additional heating during a charging/discharging reaction.
If the aluminum cathode is oxidized to deteriorate electric conductivity and the deteriorated electric conductivity increases heating due to the increased resistance, side reactions occur in the battery, which deteriorates the performance of the lithium secondary battery.